mpc5125 Freescale Semiconductor, Inc, mpc5125 Datasheet - Page 43

• Manufacturer Part Number: mpc5125
• Description: Mpc5125 Microcontroller Data Sheet
• Manufacturer: Freescale Semiconductor, Inc

The junction to ambient thermal resistance is an industry standard value, which provides a quick and easy estimation of thermal
performance. Unfortunately, there are two values in common usage: the value determined on a single-layer board, and the value
obtained on a board with two planes. For packages such as the PBGA, these values can be different by a factor of two. Which
value is correct depends on the power dissipated by other components on the board. The value obtained on a single-layer board
is appropriate for the tightly packed printed circuit board. The value obtained on the board with the internal planes is usually
appropriate if the board has low power dissipation and the components are well separated.
Historically, the thermal resistance has frequently been expressed as the sum of a junction to case thermal resistance and a case
to ambient thermal resistance:
where:
R
thermal resistance, R
arrangement on printed circuit board, or change the thermal dissipation on the printed circuit board surrounding the device. This
description is most useful for ceramic packages with heat sinks where some 90% of the heat flow is through the case to the heat
sink to ambient. For most packages, a better model is required.
A more accurate thermal model can be constructed from the junction to board thermal resistance and the junction to case thermal
resistance. The junction to case covers the situation where a heat sink is used or where a substantial amount of heat is dissipated
from the top of the package. The junction to board thermal resistance describes the thermal performance when most of the heat
is conducted to the printed circuit board. This model can be used for hand estimations or for a computational fluid dynamics
(CFD) thermal model.
To determine the junction temperature of the device in the application after prototypes are available, the thermal characterization
parameter (Ψ
the package case using the following equation:
where:
The thermal characterization parameter is measured per JESD51-2 specification using a 40-gauge type T thermocouple epoxied
to the top center of the package case. The thermocouple should be positioned so that the thermocouple junction rests on the
package. A small amount of epoxy is placed over the thermocouple junction and over approximately one mm of wire extending
from the junction. The thermocouple wire is placed flat against the package case to avoid measurement errors caused by cooling
effects of the thermocouple wire.
4.2
The MPC5125 system requires a system-level clock input SYS_XTALI. This clock input may be driven directly from an
external oscillator or with a crystal using the internal oscillator.
There is a separate oscillator for the independent real-time clock (RTC) system.
The MPC5125 clock generation uses two phase-locked loop (PLL) blocks.
Freescale Semiconductor
θJC
is device related and cannot be influenced by the user. You control the thermal environment to change the case to ambient
R
R
R
T
Ψ
P
D
T
θJA
θJC
θCA
JT
Oscillator and PLL Electrical Characteristics
= thermocouple temperature on top of package ( º C )
= power dissipation in package ( W )
JT
= thermal characterization parameter ( º C / W )
= junction to case thermal resistance ( º C / W )
= junction to ambient thermal resistance ( º C / W )
) can be used to determine the junction temperature with a measurement of the temperature at the top center of
= case to ambient thermal resistance ( º C / W )
θCA
. For instance, you can change the air flow around the device, add a heat sink, change the mounting
MPC5125 Microcontroller Data Sheet, Rev. 3
T
R
J
θ
JA
= T
= R
T
+ (
θ
Ψ
JC
JT
+ R
×
θ
P
CA
D
)
Electrical and Thermal Characteristics
Eqn. 4
Eqn. 5
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